Trialkoxysilanes, especially trimethoxysilane and triethoxysilane, are used in the production of silane coupling agents. One method of synthesis of trialkoxysilanes is directly from silicon and an alcohol. This method is known variously in the art as the Direct Synthesis, the Direct Reaction, the Direct Process or the Rochow Reaction.
For trialkoxysilanes, Direct Synthesis is most conveniently performed in slurry reactors. In a slurry reactor for the Direct Synthesis of trialkoxysilanes, catalytically-activated silicon particles are maintained in suspension in an inert, high boiling solvent and are made to react with an alcohol at an elevated temperature. This type of reaction is disclosed in U.S. Pat. Nos. 3,641,077; 3,775,457; 4,727,173; 4,761,492; 4,762,939; 4,999,446, 5,084,590; 5,103,034; 5,362,897; 5,527,938, in co-pending U.S. application Ser. No. 08/728,228 filed Oct. 10, 1996, now U.S. Pat. No. 5,728,858 and application Ser. No. 08/729,266 filed Oct. 10, 1996, now U.S. Pat. No. 5,783,720 and in Japanese Kokai Tokkyo Koho 55-28928 (1980), 55-28929 (1980), 55-76891 (1980), 57-108094 (1982) and 62-96433 (1987), 06-306083 (1994), all of which are incorporated herein by reference. Solvents disclosed in the aforementioned patents do not degrade under the activation and reaction conditions. Preferred examples are organic solvents with normal boiling points higher than about 250.degree. C. that are stable at high temperature, and that are typically used as heat exchange media. Solvents meeting these criteria include the commercial products THERMINOL.RTM.59, THERMINOL.RTM.60, THERMINOL.RTM.66, DOWTHERM.RTM.HT, MARLOTHERM.RTM.S, MARLOTHERM.RTM.L, as well as diphenyl ether, diphenyl, terphenyl, alkylated benzenes, alkylated diphenyls and alkylated terphenyls.
Tetraalkoxysilanes (also called alkyl silicates, esters of orthosilicic acid and silicon alkoxides) are prepared in slurry-phase Direct Synthesis processes wherein the solvent is often the product itself. The catalyst can be copper or a copper compound, but is usually an alkali or alkali metal salt of a high boiling alcohol. Such processes are disclosed in U.S. Pat. Nos. 3,627,807; 3,803,197; 4,113,761; 4,288,604 and 4,323,690, all incorporated herein by reference. Ethyl silicate is the tetraalkoxysilane manufactured in greatest amounts. It and its partially hydrolyzed derivatives are used for coatings, especially corrosion resistant zinc-rich coatings, and as bonding agents for a wide variety of molding and precision casting applications.
During the course of the Direct Synthesis of trialkoxysilanes, byproducts such as alkyl silicates accumulate in the solvent and contribute to an increase in viscosity, to a decline in catalytic activity and to foaming in the reaction slurry. These effects limit the long-term use of the solvent and necessitate its disposal or remediation. R. J. Ayen et al., "Better Ceramics Through Chemistry II," C. J. Brinker, D. E. Clark and D. R. Ulrich, Editors, Materials Research Society, Pittsburgh, Pa., 1986. pp 801-808, acknowledge this problem occurs with tetraethoxysilane manufacture, but they disclose no specific method of solvent or slurry disposal or recovery. U.S. Pat. No. 5,166,384 discloses the use of borates and alkali metal akoxides to precipitate the contaminants and render the solvent reusable.
Leznov, et al. in Journal of General Chemistry, USSR, 29 (1959) 1482-1487 describe an acidolysis reaction of Si--OR (R=aliphatic or aromatic group) and SiOH functional groups using carboxylic acids such as formic and acetic acids for the synthesis of cyclic and linear poly(diethylsiloxanes). Similar acidolysis reactions have also been employed in U.S. Pat. No. 2,486,992 to prepare water repellant textile finishes; in U.S. Pat. No. 2,692,838 to Produce colloidal silica suitable for coating; in U.S. Pat. No. 4,950,779 for polysiloxane synthesis; in U.S. Pat. Nos. 5,378,790; 5,412,016; 5,441,718 and in S. Sakka, et al., "Ultrastructure Processing of Advanced Ceramics," (J. D. Mackenzie and D. R. Ulrich, Editors, John Wiley & Sons, N.Y. 1988, pp 159-171) for the preparation of silicate gels. None of these references suggests a use of carboxylic acids to remove dissolved silicates and other contaminants so as to remediate used Direct Synthesis reaction solvent prior to its reuse.